This invention relates generally to heterodyne laser detection systems and, more particularly, to an improved heterodyne laser detection system wherein a laser to be detected traverses a transmission path including a high temperature transmissive component which emits thermal radiation. The improved heterodyne laser detection system is particularly applicable for use in a high speed homing missile having an infrared receiving dome or window which is aerodynamically heated to high temperatures and therefore will be described herein with reference to such application.
Due to improved technology and the relatively elementary state of development of countermeasures, lasers are replacing radar for the guidance of high speed missiles used for a variety of military applications. For example, a laser designator typically within the infrared region of the electromagnetic frequency spectrum, may be directed upon a target with the reflected laser designator signal from the target being intercepted by a homing missile. For high speed homing missiles, however, a problem may be encountered due to the high temperatures of the missile's infrared receiving dome or window which high temperatures are created by aerodynamic heating.
The total flux emitted by the infrared window is proportional to the temperature of the window to the 4th power and may even be higher if the emissivity of the infrared window increases with increasing temperature. A portion of the total power of the emissions from the window fall within the bandwidth of an infrared light detector which is utilized to sense the laser designator signal so that the detector average current rises nearly as temperature to the 4th power and the noise current rises approximately as the square root of the average current or nearly as temperature to the 2nd power. Such noise levels are in addition to ambient noise and cause standard broadband infrared detection techniques to fail.
Heterodyne type receivers have been used to provide limited bandwidth reception and, hence, overcome ambient noise as well as noise due to emissions from a heated window. However, if the bandwidth is sufficiently limited to ensure freedom from interfering noise, then the signal to be detected may not be within the receiver bandwidth due to frequency variations caused by doppler shifts and system tolerances.
Thus, the need exists for an improved laser detection or seeking system for use for example in a high speed homing missile having an infrared receiving window which is aerodynamically heated to high temperatures resulting in interferring thermal emissions from the window.